Williams, 28, has signed a two-year deal with New Zealand rugby and the Chiefs Super Rugby team for 2015-2016.
He previously played for the All Blacks from 2010 to 2012, winning the 2011 Rugby World Cup and playing in 19 tests, and played his club rugby for Canterbury and Crusaders, as well as the chiefs.
New Zealand Rugby CEO Steve Tew said: “Sonny showed in his two years with New Zealand rugby that he is an outstanding talent and was a real asset to our game. We are delighted to have agreed terms which will see him return to rugby in 2015.”
Williams was one of the stars of the recent Rugby League World Cup, his first international appearance for the Kiwis since 2007.
He returned to rugby league with the Roosters ahead of the 2013 season, helping them to the NRL title.
Earlier this week, he was stripped of his New Zealand heavyweight boxing title belts.